pidfd/cgroup-util: use fully-sized filehandle buffers

The current code assumes that FILEID_KERNFS filehandles will never grow
in size. This is not a safe assumption and userland shouldn't be trying
to guess the size of the filehandle it'll get.

We have a macro for that: MAX_HANDLE_SZ.

src/basic/cgroup-util.c

@@ -38,7 +38,7 @@
 /* The structure to pass to name_to_handle_at() on cgroupfs2 */
 typedef union {
         struct file_handle file_handle;
-        uint8_t space[offsetof(struct file_handle, f_handle) + sizeof(uint64_t)];
+        uint8_t space[MAX_HANDLE_SZ];
 } cg_file_handle;
 
 #define CG_FILE_HANDLE_INIT                                     \

src/basic/pidfd-util.c

@@ -239,7 +239,7 @@ int pidfd_get_inode_id_impl(int fd, uint64_t *ret) {
         if (file_handle_supported) {
                 union {
                         struct file_handle file_handle;
-                        uint8_t space[offsetof(struct file_handle, f_handle) + sizeof(uint64_t)];
+                        uint8_t space[MAX_HANDLE_SZ];
                 } fh = {
                         .file_handle.handle_bytes = sizeof(uint64_t),
                         .file_handle.handle_type = FILEID_KERNFS,